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Dark matter made visible

Stars, galaxies and other stuff made of ordinary matter make up only a small percentage of everything that's in the universe. Based on detailed studies of galaxies and galaxy clusters, scientists say there's six times as much mass that we can't see directly. Instead, this mysterious dark matter is detected indirectly through its gravitational effect.

The cluster, which is 2.2 billion light-years away in the constellation Virgo, serves as a gravitational lens for light coming toward us from even more distant galaxies. Like a funhouse mirror, the cluster's lens effect creates warps, magnifications and other distortions in the images of those galaxies. The Hubble team used software to analyze distortions in Hubble's view of 42 background galaxies -- and then reconstruct how much mass had to be concentrated where in order to create such distortions.

"The lensed images are like a big puzzle," Dan Coe of NASA's Jet Propulsion Laboratory said in an online statement. "Here we have figured out, for the first time, a way to arrange the mass of Abell 1689 such that it lenses all of these background galaxies to their observed positions."

The map of dark matter, superimposed on the visible-light view of the galaxy cluster, raises new puzzles. The findings confirm that Abell 1689's core has much more dark matter than scientists would have expected for a cluster its size. At some point in the evolution of the universe, scientists believe another mysterious factor known as dark energy began driving galaxies apart -- effectively stunting the growth of all galaxy clusters.

"Galaxy clusters, therefore, would had to have started forming billions of years earlier in order to build up to the numbers we see today," Coe said in a Hubble news release. "At earlier times, the universe was smaller and more densely packed with dark matter. Abell 1689 appears to have been well-fed at birth by the dense matter surrounding it in the early universe. The cluster has carried this bulk with it through its adult life to appear as we observe it today."

The research team's findings on Abell 1689 have just been published in The Astrophysical Journal, and more revelations about the interplay of dark matter and dark energy could be on the way: Coe said more conclusive results are expected from a project called CLASH (Cluster Lensing And Supernova survey with Hubble). During the CLASH survey, the space telescope will study 25 galaxy clusters for a total of one month over the next three years.